Add _ExtendAction to argparse (#8701)

_ExtendAction was added to argparse since python 3.8.

Reference of the addition to the standard library:
python/cpython@aa32a7e

stdlib/argparse.pyi

@@ -399,6 +399,10 @@ class _StoreFalseAction(_StoreConstAction):
 # undocumented
 class _AppendAction(Action): ...
 
+# undocumented
+if sys.version_info >= (3, 8):
+    class _ExtendAction(_AppendAction): ...
+
 # undocumented
 class _AppendConstAction(Action):
     if sys.version_info >= (3, 11):